Motile cilia are tubulin-based cell protrusions that beat in a coordinated fashion to mediate movement of extracellular fluids to generate directional flow. The functional importance of motile cilia are evident in a wide variety of developmental and physiological processes in complex multicellular animals. Structural and functional defects in motile cilia are associated with a variety of human conditions, including impaired mucociliary clearance in the airway, male and female infertility, cerebrospinal fluid flow, and left-right axis pattern formation. Although it has become increasingly clear that non-coding RNAs are integral components of the molecular network for development and disease, most studies on motile ciliogenesis to date have focused on the functional characterization of protein-coding genes. Our preliminary studies identified miR-34/449 miRNAs as the first non-coding RNAs that play an essential role in regulating motile ciliogenesis. The miR-34/449 miRNA family consists of six highly homologous and evolutionarily conserved miRNAs that collectively exhibit a high-level expression in all tissues containing motile cilia, and particularly, in multiciliated cells of the respiratory epithelia. The redundancy of the miR-34/449 family in the mammalian genome, combined with their dominant expression patterns in multiciliated cells, confer a robust functional regulation on motile ciliogenesis. mice deficient for all miR-34/449 miRNAs exhibited frequent postnatal mortality, strong respiratory dysfunction, and infertility. In particular, the postnatal mortality due to miR-34/449 deficiency was mostly caused by defective mucociliary clearance in the airway, which is largely due to ciliation defects in the respiratory such as mucociliary clearance in respiratory track, fertility, cerebrospinal fluid flow, and left-right axi pattern formation. Using mouse and frog genetics, cell biology and molecular biology approaches, we proposed to carefully characterize the ciliation defects in miR-34/449 deficient MCCs and animals both in development and in their physiological response to smoke exposure. In addition, we propose to elucidate the transcriptional regulation of miR-34/449 miRNAs during motile ciliogenesis, and to investigate the molecular and cellular mechanisms underlying the miR-34/449 functions during motile ciliogenesis. Taken together, these proposed studies will not only deepen our understanding on the molecular basis of motile ciliogenesis, but also provide important insights into the development of new diagnostic markers and therapeutical agents for treating respiratory conditions.